Process for Producing Water-Absorbing Polymer Particles with Low Caking Tendency and High Absorption under Pressure

ABSTRACT

A process for producing water-absorbing polymer particles with low caking tendency and high absorption under pressure, comprising polymerization of a monomer solution or suspension, drying of the resulting polymer gel, grinding, classifying, thermal surface postcrosslinking and coating with silicon dioxide, wherein the water-absorbing polymer particles have been coated, before, during or after the surface postcrosslinking with aluminum cations.

The present invention relates to a process for producing water-absorbingpolymer particles with low caking tendency and high absorption underpressure, wherein the water-absorbing polymer particles have beencoated, before, during or after the surface postcrosslinking, withsilicon dioxide and aluminum cations.

Water-absorbing polymer particles are used to produce diapers, tampons,sanitary napkins and other hygiene articles, but also as water-retainingagents in market gardening. The water-absorbing polymer particles arealso referred to as super-absorbents.

The production of water-absorbing polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, pages 71 to 103.

Water-absorbing polymer particles are typically obtained by polymerizingsuitable aqueous monomer solutions or suspensions.

The properties of the water-absorbing polymer particles can be adjusted,for example, via the amount of crosslinker used. With the increasingamount of crosslinker, the centrifuge retention capacity (CRC) falls andthe absorption under a pressure of 21.0 g/cm² (AUL0.3 psi) passesthrough a maximum.

To improve the performance properties, for example permeability of theswollen gel bed (SFC) in the diaper and absorption under a pressure of49.2 g/cm² (AUL0.7 psi), water-absorbing polymer particles are generallysurface postcrosslinked. This increases the degree of crosslinking ofthe particle surface, which allows the absorption under a pressure of49.2 g/cm² (AUL0.7 psi) and the centrifuge retention capacity (CRC) tobe decoupled at least partly. This surface postcrosslinking can becarried out in aqueous gel phase. Preferably, however, dried, ground andsieved-off polymer particles (base polymer) are surface coated with asurface postcrosslinker and thermally surface postcrosslinked.Crosslinkers suitable for this purpose are compounds which can formcovalent bonds with at least two carboxylate groups of thewater-absorbing polymer particles.

DE 35 23 617 A1 discloses a process for producing surfacepostcrosslinked water-absorbing polymer particles, wherein the free flowof the polymer particles is improved and the caking tendency is reducedby coating with silicon dioxide.

WO 00/53644 A1 and WO 00/53664 A1 describe the use of cations forimproving the centrifuge retention capacity (CRC) and permeability ofthe swollen gel bed (SFC).

It was an object of the present invention to provide an improved processfor producing water-absorbing polymer particles, especiallywater-absorbing polymer particles with low caking tendency and highabsorption under a pressure of 49.2 g/cm² (AUL0.7 psi).

The object is achieved by a process for producing water-absorbingpolymer particles by polymerizing a monomer solution or suspensioncomprising

-   a) at least one ethylenically unsaturated monomer which bears acid    groups and may be at least partly neutralized,-   b) at least one crosslinker,-   c) at least one initiator,-   d) optionally one or more ethylenically unsaturated monomers    copolymerizable with the monomers specified under a) and-   e) optionally one or more water-soluble polymers,    comprising drying the resulting aqueous polymer gel, grinding,    classifying and thermal surface postcrosslinking, which comprises    coating the water-absorbing polymer particles, before, during or    after the surface postcrosslinking, with 0.0001 to 0.25% by weight    of silicon dioxide and at least 1.5×10⁻⁶ mol/g of aluminum cations.

The silicon dioxide for use in the process according to the invention ispreferably polysilicic acids which are distinguished, according to themethod of preparation, between precipitated silicas and fumed silicas.Both variants are commercially available under the Silica FK, Sipernat®,Wessalon® (precipitated silicas) or Aerosil® (fumed silicas) names.

The silicon dioxide has a specific surface area of preferably 10 to 500m²/g, more preferably of 20 to 250 m²/g, most preferably of 50 to 200m²/g, and a mean particle size of 1 to 100 μm, more preferably of 2 to50 μm, most preferably of 5 to 20 μm.

The water-absorbing polymer particles are coated with preferably 0.001to 0.2% by weight, more preferably with 0.02 to 0.15% by weight, veryparticularly with 0.08 to 0.12% by weight, of silicon dioxide.

The water-absorbing polymer particles are preferably coated with silicondioxide after the thermal surface postcrosslinking.

The mixers usable for coating with silicon dioxide are not subject toany restriction. Advantageously, mixers with moving mixing tools, suchas screw mixers, disk mixers and paddle mixers, are used. Particularpreference is given to horizontal mixers. The distinction betweenhorizontal mixers and vertical mixers is made by the mounting of themixing shaft, i.e. horizontal mixers have a horizontally mounted mixingshaft and vertical mixers have a vertically mounted mixing shaft.Suitable horizontal mixers are, for example, Ruberg continuous flowmixers (Gebrüder Ruhberg GmbH & Co KG, Nieheim, Germany).

In the process according to the invention, the water-absorbing polymerparticles are coated with aluminum cations. Possible counterions arechloride, bromide, sulfate, hydrogensulfate, carbonate,hydrogencarbonate, nitrate, phosphate, hydrogen-phosphate,dihydrogenphosphate and preferably carboxylate, such as acetate andlactate.

Particular preference is given to using basic carboxylates, such asbasic aluminum acetate. Very particular preference is given to aluminummonoacetate (CAS No. [7360-44-3]). In the basic carboxylates, not allhydroxide groups eliminable as hydroxyl anions (OH⁻) in aqueoussolutions are replaced in the salt-forming bases by carboxylate groups.

The water-absorbing polymer particles are coated with preferably 2×10⁻⁶to 15×10⁻⁶ mol/g (moles of aluminum cations per g of water-absorbingpolymer particles), more preferably with 3×10⁻⁶ to 10×10⁻⁶ mol/g, veryparticularly with 4×10⁻⁶ to 6×10⁻⁶ mol/g, of aluminum cations.

The water-absorbing polymer particles are preferably coated withaluminum cations before the thermal surface postcrosslinking.

For the coating with the aluminum cations, the mixers suitable for sprayapplication of the surface postcrosslinker are advantageously used.

The present invention is based on the finding that the absorption undera pressure of 49.2 g/cm² (AUL0.7 psi) of water-absorbing polymerparticles falls as a result of coating with silicon dioxide. In order toachieve a sufficient reduction in the caking tendency, a considerabledecline in the absorption under a pressure of 49.2 g/cm² (AUL0.7 psi)therefore has to be accepted.

Moreover, it has been found that aluminum cations and silicon dioxideact synergistically, such that, in the presence of aluminum cations,significantly smaller amounts of silicon dioxide bring about asufficient reduction in the caking tendency.

The acid groups of the ethylenically unsaturated monomers bearing acidgroups have preferably been neutralized to an extent of 72 to 82 mol %,more preferably to an extent of 74 to 79 mol %, most preferably of 75 to77 mol %. The decline in the absorption under a pressure of 49.2 g/cm²(AUL0.7 psi) which is caused by the coating with silicon dioxidedecreases with rising degree of neutralization, while the reaction rateof the thermal surface postcrosslinking decreases simultaneously.

The present invention is particularly advantageous for water-absorbingpolymer particles with high centrifuge retention capacity (CRC). Theproperties of water-absorbing polymer particles with high centrifugeretention capacity (CRC) are particularly adversely affected by thecoating with silicon dioxide. The additional coating with aluminumcations to reduce the amount of silicon dioxide needed is thereforeparticularly beneficial in this case. The centrifuge retention capacity(CRC) of the water-absorbing polymer particles obtainable by the processaccording to the invention is therefore preferably from 35 to 45 g/g,more preferably from 36 to 40 g/g, most preferably from 35 to 37 g/g.The centrifuge retention capacity (CRC) can be adjusted by adjusting theconcentration of the crosslinker b). With rising concentration of thecrosslinker b), the centrifuge retention capacity (CRC) falls and theabsorption under a pressure of 21.0 g/cm² (AUL0.3 psi) passes through amaximum.

The production of the water-absorbing polymer particles is explained indetail hereinafter:

The water-absorbing polymer particles are produced by polymerizing amonomer solution or suspension and are typically water-insoluble.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 35 g/100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid and itaconicacid. Particularly preferred monomers are acrylic acid and methacrylicacid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturatedsulfonic acids, such as styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities can have a considerable influence on the polymerization. Theraw materials used should therefore have a maximum purity. It istherefore often advantageous to specially purify the monomers a).Suitable purification processes are described, for example, in WO2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitablemonomer a) is, for example, acrylic acid purified according to

WO 2004/035514 A1 comprising 99.8460% by weight of acrylic acid, 0.0950%by weight of acetic acid, 0.0332% by weight of water, 0.0203% by weightof propionic acid, 0.0001% by weight of furfurals, 0.0001% by weight ofmaleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% byweight of hydroquinone monomethyl ether.

The proportion of acrylic acid and/or salts thereof in the total amountof monomers a) is preferably at least 50 mol %, more preferably at least90 mol %, most preferably at least 95 mol %.

The monomers a) typically comprise polymerization inhibitors, preferablyhydroquinone half ethers, as storage stabilizers.

The monomer solution comprises preferably up to 250 ppm by weight,preferably at most 130 ppm by weight, more preferably at most 70 ppm byweight, preferably at least 10 ppm by weight, more preferably at least30 ppm by weight, especially around 50 ppm by weight, of hydroquinonehalf ether, based in each case on the unneutralized monomer a). Forexample, the monomer solution can be prepared by using an ethylenicallyunsaturated monomer bearing acid groups with an appropriate content ofhydroquinone half ether.

Preferred hydroquinone half ethers are hydroquinone monomethyl ether(MEHQ) and/or alpha-tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groupssuitable for crosslinking. Such groups are, for example, ethylenicallyunsaturated groups which can be polymerized free-radically into thepolymer chain, and functional groups which can form covalent bonds withthe acid groups of the monomer a). In addition, polyvalent metal saltswhich can form coordinate bonds with at least two acid groups of themonomer a) are also suitable as crosslinkers b).

Crosslinkers b) are preferably compounds having at least twopolymerizable groups which can be polymerized free-radically into thepolymer network. Suitable crosslinkers b) are, for example, ethyleneglycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycoldiacrylate, allyl methacrylate, trimethylolpropane triacrylate,triallylamine, tetraallylammonium chloride, tetraallyloxyethane, asdescribed in EP 0 530 438 A1, di- and triacrylates, as described in EP 0547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 103 31 450A1, mixed acrylates which, as well as acrylate groups, comprise furtherethylenically unsaturated groups, as described in DE 103 31 456 A1 andDE 103 55 401 A1, or crosslinker mixtures, as described, for example, inDE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/032962A2.

Preferred crosslinkers b) are pentaerythrityl triallyl ether,tetraalloxyethane, methylene-bismethacrylamide, 15-tuply ethoxylatedtrimethylolpropane triacrylate, polyethylene glycol diacrylate,trimethylolpropane triacrylate and triallylamine.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample, in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol, especially thetriacrylate of 3-tuply ethoxylated glycerol.

The initiators c) may be all compounds which generate free radicalsunder the polymerization conditions, for example thermal initiators,redox initiators, photo-initiators. Suitable redox initiators are sodiumperoxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid, sodiumperoxodisulfate/sodium bisulfite and hydrogen peroxide/sodium bisulfite.Preference is given to using mixtures of thermal initiators and redoxinitiators, such as sodium peroxodisulfate/hydrogen peroxide/ascorbicacid. The reducing component used is, however, preferably a mixture ofthe sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium saltof 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixturesare obtainable as Brüggolite® FF6 and Brüggolite® FF7 (BrüggemannChemicals; Heilbronn; Germany).

Ethylenically unsaturated monomers d) copolymerizable with theethylenically unsaturated monomers a) bearing acid groups are, forexample, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethylmethacrylate, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, dimethylaminopropyl acrylate, diethyl-aminopropyl acrylate,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.

The water-soluble polymers e) used may be polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, modified cellulose,such as methylcellulose or hydroxyethyl-cellulose, gelatin, polyglycolsor polyacrylic acids, preferably starch, starch derivatives and modifiedcellulose.

Typically, an aqueous monomer solution is used. The water content of themonomer solution is preferably from 40 to 75% by weight, more preferablyfrom 45 to 70% by weight, most preferably from 50 to 65% by weight. Itis also possible to use monomer suspensions, i.e. monomer solutions withexcess monomer a), for example sodium acrylate. With rising watercontent, the energy requirement in the subsequent drying rises, and,with falling water content, the heat of polymerization can only beremoved inadequately.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. Before the polymerization, the monomer solution cantherefore be freed of dissolved oxygen by inertization, i.e. flowing aninert gas through, preferably nitrogen or carbon dioxide. The oxygencontent of the monomer solution is preferably lowered before thepolymerization to less than 1 ppm by weight, more preferably to lessthan 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

Suitable reactors are, for example, kneading reactors or belt reactors.In the kneader, the polymer gel formed in the polymerization of anaqueous monomer solution or suspension is comminuted continuously by,for example, contrarotatory stirrer shafts, as described in WO2001/038402 A1. Polymerization on a belt is described, for example, inDE 38 25 366 A1 and U.S. Pat. No. 6,241,928. Polymerization in a beltreactor forms a polymer gel, which has to be comminuted in a furtherprocess step, for example in an extruder or kneader.

To improve the drying properties, the comminuted polymer gel obtained bymeans of a kneader can additionally be extruded.

The acid groups of the resulting polymer gels have been partiallyneutralized. Neutralization is preferably carried out at the monomerstage. This is typically done by mixing in the neutralizing agent as anaqueous solution or preferably also as a solid. The customaryneutralizing agents can be used, preferably alkali metal hydroxides,alkali metal oxides, alkali metal carbonates or alkali metalhydrogencarbonates and also mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonium salts. Particularly preferredalkali metals are sodium and potassium, but very particular preferenceis given to sodium hydroxide, sodium carbonate or sodiumhydrogencarbonate and also mixtures thereof.

However, it is also possible to carry out neutralization after thepolymerization, at the stage of the polymer gel formed in thepolymerization. It is also possible to neutralize up to 40 mol %,preferably from 10 to 30 mol % and more preferably from 15 to 25 mol %of the acid groups before the polymerization by adding a portion of theneutralizing agent actually to the monomer solution and setting thedesired final degree of neutralization only after the polymerization, atthe polymer gel stage. When the polymer gel is neutralized at leastpartly after the polymerization, the polymer gel is preferablycomminuted mechanically, for example by means of an extruder, in whichcase the neutralizing agent can be sprayed, sprinkled or poured on andthen carefully mixed in. To this end, the gel mass obtained can berepeatedly extruded for homogenization.

The polymer gel is then dried with a forced air belt dryer until theresidual moisture content is preferably from 0.5 to 15% by weight, morepreferably from 1 to 10% by weight, most preferably from 2 to 8% byweight, the residual moisture content being determined by the EDANArecommended test method No. WSP 230.2-05 “Moisture Content”. In the caseof too high a residual moisture content, the dried polymer gel has toolow a glass transition temperature T_(g) and can be processed furtheronly with difficulty. In the case of too low a residual moisturecontent, the dried polymer gel is too brittle and, in the subsequentcomminution steps, undesirably large amounts of polymer particles withan excessively low particle size are obtained (fines). The solidscontent of the gel before the drying is preferably from 25 to 90% byweight, more preferably from 35 to 70% by weight, most preferably from40 to 60% by weight.

Thereafter, the dried polymer gel is ground and classified, and theapparatus used for grinding may typically be single- or multistage rollmills, preferably two- or three-stage roll mills, pin mills, hammermills or vibratory mills.

The mean particle size of the polymer particles removed as the productfraction is preferably at least 200 μm, more preferably from 250 to 600μm, very particularly from 300 to 500 μm. The mean particle size of theproduct fraction may be determined by means of the EDANA recommendedtest method No. WSP 220.2-05 “Particle Size Distribution”, where theproportions by mass of the screen fractions are plotted in cumulatedform and the mean particle size is determined graphically. The meanparticle size here is the value of the mesh size which gives rise to acumulative 50% by weight.

The proportion of particles with a particle size of at least 150 μm ispreferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

Polymer particles with too small a particle size lower the permeabilityof the swollen gel bed (SFC). The proportion of excessively smallpolymer particles (fines) should therefore be small.

Excessively small polymer particles are therefore typically removed andrecycled into the process. This is preferably done before, during orimmediately after the polymerization, i.e. before the drying of thepolymer gel. The excessively small polymer particles can be moistenedwith water and/or aqueous surfactant before or during the recycling.

It is also possible to remove excessively small polymer particles inlater process steps, for example after the surface postcrosslinking oranother coating step. In this case, the excessively small polymerparticles recycled are surface postcrosslinked or coated in another way,for example with fumed silica.

When a kneading reactor is used for the polymerization, the excessivelysmall polymer particles are preferably added during the last third ofthe polymerization.

When the excessively small polymer particles are added at a very earlystage, for example actually to the monomer solution, this lowers thecentrifuge retention capacity (CRC) of the resulting water-absorbingpolymer particles. However, this can be compensated, for example, byadjusting the amount of crosslinker b) used.

When the excessively small polymer particles are added at a very latestage, for example not until within an apparatus connected downstream ofthe polymerization reactor, for example an extruder, the excessivelysmall polymer particles can be incorporated into the resulting polymergel only with difficulty. Excessively small polymer particles which havebeen insufficiently incorporated, however, become detached again fromthe dried polymer gel during the grinding, and are therefore removedagain in the classification and increase the amount of excessively smallpolymer particles to be recycled.

The proportion of particles having a particle size of at most 850 μm ispreferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

The proportion of polymer particles with a particle size of at most 600μm is preferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

Polymer particles with too great a particle size lower the swell rate.The proportion of excessively large polymer particles should thereforelikewise be small.

Excessively large polymer particles are therefore typically removed andrecycled into the grinding of the dried polymer gel.

To further improve the properties, the polymer particles are surfacepostcrosslinked. Suitable surface postcrosslinkers are compounds whichcomprise groups which can form covalent bonds with at least twocarboxylate groups of the polymer particles. Suitable compounds are, forexample, polyfunctional amines, polyfunctional amido amines,polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described inDE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, orβ-hydroxyalkylamides, as described in DE 102 04 938 A1 and U.S. Pat. No.6,239,230.

Additionally described as suitable surface postcrosslinkers are cycliccarbonates in DE 40 20 780 C1, 2-oxazolidone and its derivatives, suchas 2-hydroxyethyl-2-oxazolidone in DE 198 07 502 A1, bis- andpoly-2-oxazolidinones in DE 198 07 992 C1, 2-oxotetrahydro-1,3-oxazineand its derivatives in DE 198 54 573 A1, N-acyl-2-oxazolidones in DE 19854 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amide acetals inDE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327 A2 andmorpholine-2,3-dione and its derivatives in WO 2003/031482 A1.

Preferred surface postcrosslinkers are ethylene carbonate, ethyleneglycol diglycidyl ether, reaction products of polyamides withepichlorohydrin, and mixtures of propylene glycol and 1,4-butanediol.

Very particularly preferred surface postcrosslinkers are2-hydroxyethyloxazolidin-2-one, oxazolidin-2-one and 1,3-propanediol.

In addition, it is also possible to use surface postcrosslinkers whichcomprise additional polymerizable ethylenically unsaturated groups, asdescribed in DE 37 13 601 A1.

The amount of surface postcrosslinker is preferably from 0.001 to 2% byweight, more preferably from 0.02 to 1% by weight, most preferably from0.05 to 0.2% by weight, based in each case on the polymer particles.

In a preferred embodiment of the present invention, before, during orafter the surface postcrosslinking, in addition to the aluminum cations,further polyvalent cations are applied to the particle surface.

The further polyvalent cations usable in the process according to theinvention are, for example, divalent cations such as the cations ofzinc, magnesium, calcium, iron and strontium, trivalent cations such asthe cations of iron, chromium, rare earths and manganese, tetravalentcations such as the cations of titanium and zirconium. Possiblecounterions are chloride, bromide, sulfate, hydrogensulfate, carbonate,hydrogencarbonate, nitrate, phosphate, hydrogenphosphate,dihydrogenphosphate and carboxylate, such as acetate and lactate. Apartfrom metal salts, it is also possible to use polyamines as furtherpolyvalent cations.

The amount of further polyvalent cation used is, for example, 0.001 to1.5% by weight, preferably 0.005 to 1% by weight, more preferably 0.02to 0.8% by weight, based in each case on the polymer particles.

The surface postcrosslinking is typically performed in such a way that asolution of the surface postcrosslinker is sprayed onto the driedpolymer particles. After the spraying, the polymer particles coated withsurface postcrosslinker are dried thermally, and the surfacepostcrosslinking reaction can take place either during or after thedrying.

The spraying of a solution of the surface postcrosslinker is preferablyperformed in mixers with moving mixing tools, such as screw mixers, diskmixers and paddle mixers. Particular preference is given to horizontalmixers such as paddle mixers, very particular preference to verticalmixers. The distinction between horizontal mixers and vertical mixers ismade by the position of the mixing shaft, i.e. horizontal mixers have ahorizontally mounted mixing shaft and vertical mixers a verticallymounted mixing shaft. Suitable mixers are, for example, horizontalPflugschar® plowshare mixers (Gebr. Lödige Maschinenbau GmbH; Paderborn;Germany), Vrieco-Nauta Continuous Mixers (Hosokawa Micron By;Doetinchem; the Netherlands), Processall Mixmill Mixers (ProcessallIncorporated; Cincinnati; US) and Schugi Flexomix® (Hosokawa Micron BV;Doetinchem; the Netherlands). However, it is also possible to spray onthe surface postcrosslinker solution in a fluidized bed.

The surface postcrosslinkers are typically used in the form of anaqueous solution. The penetration depth of the surface postcrosslinkerinto the polymer particles can be adjusted via the content of nonaqueoussolvent and total amount of solvent.

When exclusively water is used as the solvent, a surfactant isadvantageously added. This improves the wetting behavior and reduces thetendency to form lumps. However, preference is given to using solventmixtures, for example isopropanol/water, 1,3-propanediol/water andpropylene glycol/water, where the mixing ratio in terms of mass ispreferably from 20:80 to 40:60.

The temperature of the water-absorbing polymer particles in the dryer ispreferably from 100 to 250° C., more preferably from 130 to 220° C.,most preferably from 150 to 200° C. The residence time in the dryer ispreferably from 10 to 120 minutes, more preferably from 10 to 90minutes, most preferably from 30 to 60 minutes. The filling of the dryeris preferably from 30 to 80%, more preferably from 40 to 75%, mostpreferably from 50 to 70%. The filling of the dryer can be adjusted viathe height of the outflow weir.

Subsequently, the surface postcrosslinked polymer particles can beclassified again to remove excessively small and/or excessively largepolymer particles which are recycled into the process.

The properties of the surface postcrosslinked polymer particles can beimproved further by coating or remoisturizing.

The remoisturizing is performed preferably at 30 to 80° C., morepreferably at 35 to 70° C., most preferably at 40 to 60° C. Atexcessively low temperatures the water-absorbing polymer particles tendto form lumps, and at higher temperatures water already evaporatesnoticeably. The amount of water used for remoisturizing is preferablyfrom 1 to 10% by weight, more preferably from 2 to 8% by weight, mostpreferably from 3 to 5% by weight. The remoisturizing increases themechanical stability of the polymer particles and reduces the tendencythereof to static charging.

Suitable coatings for improving the swell rate and the permeability ofthe swollen gel bed (SFC) are, for example, inorganic inert substancessuch as water-insoluble metal salts, organic polymers, cationic polymersand di- or polyhydric metal cations. Suitable coatings for dust bindingare, for example, polyols.

The present invention further provides the water-absorbing polymerparticles obtainable by the process according to the invention.

The present invention further provides water-absorbing polymer particlescomposed of crosslinked polymers bearing acid groups which have beenneutralized to an extent of 70 to 85 mol %, comprising 0.0001 to 0.25%by weight of silicon dioxide and at least 1.5×10⁻⁶ mol/g of aluminumcations.

The water-absorbing polymer particles comprise preferably 0.001 to 0.2%by weight, more preferably 0.02 to 0.15% by weight, very particularly0.08 to 0.12% by weight, of silicon dioxide.

The water-absorbing polymer particles comprise preferably 2×10⁻⁶ to15×10⁻⁶ mol/g, more preferably 3×10⁻⁶ to 10×10⁻⁶ mol/g, veryparticularly 4×10⁻⁶ to 6×10⁻⁶ mol/g, of aluminum cations.

The acid groups of the water-absorbing polymer particles have preferablybeen neutralized to an extent of 72 to 82 mol %, more preferably to anextent of 74 to 79 mol %, most preferably of 75 to 77 mol %.

The inventive water-absorbing polymer particles have a centrifugeretention capacity (CRC) of preferably from 25 to 45 g/g, morepreferably from 30 to 40 g/g, most preferably from 35 to 37 g/g. Thecentrifuge retention capacity (CRC) is determined by EDANA recommendedtest method No. WSP 241.2-05 “Centrifuge Retention Capacity”.

The inventive water-absorbing polymer particles have a moisture contentof preferably 0 to 15% by weight, more preferably 0.2 to 10% by weight,most preferably 0.5 to 8% by weight, the moisture content beingdetermined by EDANA recommended test method No. WSP 230.2-05 “MoistureContent”.

The inventive water-absorbing polymer particles have an absorption undera pressure of 49.2 g/cm² (AUL0.7 psi) of typically at least 15 g/g,preferably at least 18 g/g, preferentially at least 20 g/g, morepreferably at least 21 g/g, most preferably at least 22 g/g. Theabsorption under a pressure of 49.2 g/cm² (AUL0.7 psi) of thewater-absorbing polymer particles is typically less than 30 g/g. Theabsorption under a pressure of 492 g/cm² (AUL0.7 psi) is determinedanalogously to EDANA recommended test method No. WSP 242.2-05“Absorption under Pressure”, except setting a pressure of 49.2 g/cm²instead of a pressure of 21.0 g/cm².

The present invention further provides hygiene articles comprisinginventive water-absorbing polymer particles. The hygiene articlestypically comprise a water-impervious backside, a water-pervious topsideand, in between, an absorbent core composed of the inventive polymerparticles and cellulose fibers. The proportion of the inventive polymerparticles in the absorbent core is preferably 20 to 100% by weight,preferentially 50 to 100% by weight.

The water-absorbing polymer particles are tested by means of the testmethods described below.

Methods:

The measurements should, unless stated otherwise, be carried out at anambient temperature of 23±2° C. and a relative air humidity of 50±10%.The water-absorbing polymer particles are mixed thoroughly before themeasurement.

Centrifuge Retention Capacity

The centrifuge retention capacity (CRC) is determined by EDANArecommended test method No. WSP 214.2-05 “Centrifuge RetentionCapacity”.

Absorption Under a Pressure of 49.2 g/cm²

The absorption under a pressure of 49.2 g/cm² (AUL0.7 psi) is determinedanalogously to EDANA recommended test method No. WSP 242.2-05“Absorption under Pressure”, with a pressure setting of 49.2 g/cm²(AUL0.7 psi) instead of 21.0 g/cm² (AUL0.3 psi).

Caking

To determine the caking tendency, the empty weight (Wd) of an aluminumdish (diameter 5.7 cm) is determined. Subsequently, 5 g ofwater-absorbing polymer particles are weighed into the aluminum dish,and the aluminum dish covered with the water-absorbing polymer particlesis placed in a climate-controlled cabinet at 40° C. and 80% relative airhumidity for one hour.

After the storage, the aluminum dish covered with the water-absorbingpolymer particles is weighed and the weight (WHYD) is noted.

Subsequently, the empty weight (WPAN) of a screen (mesh size 1.7 mm anddiameter 76.2 mm) is determined, and the water-absorbing polymerparticles are placed onto the screen and screened by means of avibration screening machine (amplitude 0.2 cm) for one minute.

After the screening, the screen covered with the water-absorbing polymerparticles is weighed and the weight (WUNC) is noted.

The caking tendency is calculated by

${Caking} = \frac{{WUNC} - {WPAN}}{{WHYD} - {Wd}}$

The caking tendency reports the proportion by weight of water-absorbingpolymer particles which have formed lumps, and the lower the value is,the lower the caking is.

The EDANA test methods are obtainable, for example, from EDANA, AvenueEugene Plasky 157, B-1030 Brussels, Belgium.

EXAMPLES

5009 g of a 37.3% by weight aqueous sodium acrylate solution were mixedwith 477 g of acrylic acid and 430 g of water, and inertized withnitrogen. This mixture was introduced into a nitrogen-inertized Werner &Pfleiderer LUK 8,0 K2 kneader (2 Sigma shafts), and admixed successivelywith 9.0 g of polyethylene glycol diacrylate (diacrylate of apolyethylene glycol with a mean molar mass of 400 g/mol), 4.4 g of a1.0% by weight aqueous ascorbic acid solution, 18.1 g of a 15% by weightaqueous sodium persulfate solution and 3.9 g of a 3% by weight aqueoushydrogen peroxide solution. The kneader was stirred at maximum speed(approx. 98 rpm of the faster shaft, approx. 49 rpm of the slower shaft,ratio approx. 2:1). Immediately after the addition of hydrogen peroxide,the kneader jacket was heated with heat carrier at 80° C. On attainmentof the maximum temperature, the jacket heating was switched off and themixture was allowed to react in the kneader for a further 15 minutes.The resulting polymer gel was cooled to 65° C. and emptied. The polymergel was dried at 175° C. for 75 minutes with a loading of 700 g permetal sheet in a forced-air drying cabinet. After grinding three timesin a roll mill (Gebr. Baumeister LRC 125/70, gap widths 1000 μm, 600 μm,400 μm), the polymer particles were screened off to a screening fractionbetween 850 and 100 μm.

The resulting water-absorbing polymer particles had a degree ofneutralization of 75 mol %. Water-absorbing polymer particles with adegree of neutralization of 72 mol % and 77 mol % were obtainedanalogously by adjusting the amounts of sodium acrylate solution,acrylic acid and water, while the solids content of the monomer solutionwas kept constant.

1000 g of these polymer particles were transferred into a Gebr. Lödigelaboratory mixer (M5R). At approx. 23° C., a mixture of 0.6 g of2-hydroxyethyloxazolidin-2-one, 0.6 g of 1,3-propanediol, 16.5 g ofwater, 11.5 g of 2-propanol and an aqueous aluminum salt solution (1 g,2 g, 4 g or 6 g of 25% by weight aluminum lactate [18917-91-4], 2.4 g or3.6 g of 26.8% by weight aluminum sulfate [10043-01-3] or 1.6 g or 2.4 gof 17.4% by weight basic aluminum acetate [7360-44-3]) were sprayed onvia a nozzle. The sprayed polymer particles were transferred intoanother Gebr. Lödige laboratory mixer which was heated rapidly to 170°C., 175° C. or 180° C. and kept at this temperature for 60 minutes.After cooling, the surface postcrosslinked polymer particles werescreened off to a screening fraction between 850 and 100 μm.

In a 500 ml glass bottle, 100 g of surface postcrosslinked polymerparticles were optionally mixed with 0.1 g, 0.2 g or 0.3 g of silicondioxide (Sipernat® D17) for 15 minutes by means of a Turbula® T2F mixer(Willy A. Bachofen A G Maschinenfabrik; Muttenz; Switzerland) at 45 rpm.

The resulting water-absorbing polymer particles were analyzed. Theresults are compiled in the following tables:

TABLE 1 Degree of neutralization 72 mol % and surface postcrosslinkingat 170° C. before SiO₂ addition after SiO₂ addition Al³⁺ Caking CRCAUL0.7 psi SiO₂ Caking CRC AUL0.7 psi Ex. [10⁻⁶ mol/g] [%] [g/g] [g/g][% by wt.] [%] [g/g] [g/g]  1*) 0 36.3 22.9 0.1 32.8  2*) 0.2 7.0  3*)0.3 5.1  4*) 0.85 35.0 23.8 0.1 30.6 35.8 19.8  5*) 0.2 10.7 36.0 17.3 6*) 0.3 3.0 35.3 17.0  7*) 1.7 34.5 23.2 0.1 19.9 36.4 19.0  8*) 0.217.1 35.4 17.3  9*) 0.3 1.1 35.4 17.4 10 3.4 68.1 35.7 24.4 0.1 0.8 36.320.0 11 0.2 0.7 36.5 18.4 12 0.3 0.9 36.2 17.8 13 5.1 65.0 35.9 25.0 0.10.9 35.8 20.7 14 0.2 1.0 35.6 18.3 15 0.3 1.1 35.2 18.3 *)Comparativeexample

TABLE 2 Degree of neutralization 75 mol % and surface postcrosslinkingat 175° C. before SiO₂ addition after SiO₂ addition Al³⁺ Caking CRCAUL0.7 psi SiO₂ Caking CRC AUL0.7 psi Ex. [10⁻⁶ mol/g] [%] [g/g] [g/g][% by wt.] [%] [g/g] [g/g] 16*) 0 99.5 35.9 23.0 0.1 61.3 36.1 20.8 17*)0.2 34.6 35.8 20.1 18*) 0.3 8.9 35.6 19.6 19 3.4 97.6 35.8 25.6 0.1 1.035.9 22.3 20 0.2 0.7 35.6 21.9 21 0.3 0.6 35.2 21.3 22 5.1 87.2 34.824.6 0.1 0.7 35.4 23.0 23 0.2 0.3 35.4 21.4 24 0.3 0.6 34.3 20.9*)Comparative example

TABLE 3 Degree of neutralization 77 mol % and surface postcrosslinkingat 175° C. before SiO₂ addition after SiO₂ addition Al³⁺ Caking CRCAUL0.7 psi SiO₂ Caking CRC AUL0.7 psi Ex. [10⁻⁶ mol/g] [%] [g/g] [g/g][% by wt.] [%] [g/g] [g/g] 25*) 0 100.0 36.9 21.6 0.1 10.6 37.4 16.526*) 0.2 6.3 37.3 14.3 27 3.4 92.3 37.9 21.3 0.1 0.6 38.2 16.6 28 0.20.3 38.6 14.1 29 5.1 66.4 37.2 19.6 0.1 1.1 38.7 16.4 30 0.2 0.8 37.314.4 *)Comparative example

TABLE 4 Degree of neutralization 77 mol % and surface postcrosslinkingat 180° C. before SiO₂ addition after SiO₂ addition Al³⁺ Caking CRCAUL0.7 psi SiO₂ Caking CRC AUL0.7 psi Ex. [10⁻⁶ mol/g] [%] [g/g] [g/g][% by wt.] [%] [g/g] [g/g] 31*) 0 99.9 35.3 22.1 0.1 9.8 35.6 19.2 323.4 94.2 35.9 25.3 0.1 1.0 36.7 20.1 33 0.2 0.0 36.7 19.3 34 5.1 51.734.5 23.2 0.1 0.8 35.9 22.0 35 0.2 0.5 35.6 19.9 *)Comparative example

The results in tables 1 to 4 demonstrate the synergistic effect ofaluminum cations and silicon dioxide in the reduction of the cakingtendency of water-absorbing polymer particles.

In addition, the results show that, in the event of an increase in thedegree of neutralization by 2 to 3 mol %, the temperature in the thermalsurface postcrosslinking has to be raised by approx. 5° C. in order toobtain comparable values for the centrifuge retention capacity (CRC) andthe absorption under pressure (AUL0.7 psi).

In addition, the decline in the absorption under pressure (AUL0.7 psi)at a degree of neutralization of 75 mol %, which is caused by thecoating with silicon dioxide, passes through a minimum.

TABLE 5 Degree of neutralization 72 mol %, surface postcrosslinking at170° C. and 3.4 × 10⁻⁶ mol/g Al³⁺ before SiO₂ addition after SiO₂addition Aluminum Caking CRC AUL0.7 psi SiO₂ Caking CRC AUL0.7 psi Ex.salt [%] [g/g] [g/g] [% by wt.] [%] [g/g] [g/g] 36 Lactate 35.7 24.4 0.10.8 36.3 20.0 37 0.2 0.7 36.5 18.4 38 Sulfate 37.3 19.8 0.1 26.7 38.015.3 39 0.2 3.3 37.5 13.5 40 Acetate 37.5 22.6 0.1 0.9 37.3 20.2 41 0.20.5 36.8 19.4 *) Comparative example

TABLE 6 Degree of neutralization 72 mol %, surface postcrosslinking at170° C. and 5.1 × 10⁻⁶ mol/g Al³⁺ before SiO₂ addition after SiO₂addition Aluminum Caking CRC AUL0.7 psi SiO₂ Caking CRC AUL0.7 psi Ex.salt [%] [g/g] [g/g] [Gew.-%] [%] [g/g] [g/g] 42 Lactate 35.9 25.0 0.10.9 35.8 20.7 43 0.2 1.0 35.6 18.3 44 Sulfate 38.7 18.5 0.1 5.0 37.714.4 45 0.2 0.0 38.0 13.4 46 Acetate 36.5 21.3 0.1 0.6 37.9 19.7 47 0.20.4 37.3 20.2

The results in tables 5 and 6 show the differences when using differentaluminum salts. The coating with aluminum lactate leads towater-absorbing polymer particles with the highest absorption underpressure (AUL0.7 psi). The water-absorbing polymer particles coated withbasic aluminum acetate show, in contrast, the smallest decline in theabsorption under pressure (AUL0.7 psi) when they are subsequently coatedwith silicon dioxide.

1. A process for producing water-absorbing polymer particles by polymerizing an aqueous monomer solution or suspension comprising a) at least one ethylenically unsaturated monomer which bears an acid group, which has been neutralized to an extent of 70 to 85 mol %, b) at least one crosslinker, c) at least one initiator, d) optionally one or more ethylenically unsaturated monomer copolymerizable with the monomer specified under a), and e) optionally one or more water-soluble polymer, comprising drying a resulting aqueous polymer gel, grinding, classifying and thermal surface postcrosslinking to provide water-absorbing polymer particles, which comprises coating the water-absorbing polymer particles, before, during, or after the surface postcrosslinking, with 0.0001 to 0.25% by weight of silicon dioxide and at least 1.5×10⁻⁶ mol/g of aluminum cations.
 2. The process according to claim 1, wherein the water-absorbing polymer particles are coated with 0.08 to 0.12% by weight of silicon dioxide.
 3. The process according to claim 1, wherein the water-absorbing polymer particles are coated with silicon dioxide after the surface postcrosslinking.
 4. The process according to claim 1, wherein the water-absorbing polymer particles are coated with 4×10⁻⁶ to 6×10⁻⁶ mol/g of aluminum cations.
 5. The process according to claim 1, wherein the water-absorbing polymer particles are coated with aluminum cations before the surface postcrosslinking.
 6. The process according to claim 1, wherein 75 to 77 mol % of the acid groups of the water-absorbing polymer particles have been neutralized.
 7. The process according to claim 1, wherein the amount of crosslinker b) is adjusted such that the water-absorbing polymer particles have a centrifuge retention capacity of 35 to 37 g/g.
 8. Water-absorbing polymer particles produced by a process according to claim
 1. 9. Water-absorbing polymer particles comprising crosslinked polymers bearing acid groups which have been neutralized to an extent of 70 to 85 mol %, comprising 0.0001 to 0.25% by weight of silicon dioxide and at least 1.5×10⁻⁶ mol/g of aluminum cations.
 10. The polymer particles according to claim 9, wherein the water-absorbing polymer particles comprise from 0.08 to 0.12% by weight of silicon dioxide.
 11. The polymer particles according to claim 9, wherein the water-absorbing polymer particles comprise 4×10⁻⁶ to 6×10⁻⁶ mol/g of aluminum cations.
 12. The polymer particles according to claim 9, wherein 75 to 77 mol % of the acid groups of the water-absorbing polymer particles have been neutralized.
 13. The polymer particles according to claim 9, wherein the water-absorbing polymer particles have a centrifuge retention capacity of from 35 to 37 g/g.
 14. A hygiene article comprising water-absorbing polymer particles according to claim
 9. 